In many video systems, it is often necessary to effect a conversion of a digital signal from a first sampling frequency to a second sampling frequency, according to the image format required by the reception device. The conversion results in an enlargement or reduction of the original image corresponding to a up-sampling or a down-sampling of said image.
Such a conversion can be implemented by means of a finite impulse response filter FIR with a polyphase structure. Canadian patent number 2,144,111 describes a conversion method using such a filter. The term polyphase indicates a periodic representation of the phase differences between a sample of the input digital signal and a sample of the output digital signal. These phase differences are calculated according to the inverse of a zoom factor, the zoom factor representing the ratio between the number of samples of the output signal and the number of samples of the input signal. The polyphase filter functions in direct mode for an enlargement of the image, i.e. for a zoom factor greater than 1, and in transposed mode for a reduction of the image, i.e. for a zoom factor lower than 1.
A conventional polyphase filter comprises a convoluter able to supply an output digital signal sampled at a frequency f2 from an input digital signal sampled at a frequency f1 and a set of filtering coefficients. A memory associates with each possible phase difference a set of n filtering coefficients. The convoluter comprises shift registers for temporarily storing the samples of the input signal in the direct operating mode of the polyphase filter or the samples of the output signal in its transposed operating mode. Calculation means calculate on the one hand the phase difference and on the other hand the shift signal of the shift registers.
A polyphase filter of this type is specifically designed firstly for a predetermined number of n filtering coefficients per set and secondly for a direct or transposed operating mode. The shift signal is calculated by successive incrementation of the inverse of the zoom factor, this calculation being carried out by a calculation unit dedicated to a given polyphase filter. Consequently said calculation unit is designed specifically for this polyphase filter and cannot be used for another polyphase filter.